Your Turbo allocation is the number of Gbs you are given that are unshaped (meaning very fast). If you are on a subscription with 5 Turbo Gbs, that means
your first 5 Gbs will be high-speed. Once those are used up, your traffic will be shaped according to your subscription (see details here).

If you extend or upgrade your subscription, your Turbo usage gets reset when your subscription rolls over to the new month.

Warning messages will be sent to your primary email address when you are close to using up your Turbo Gbs, but if that happens, don't worry. You will still be able to access the Internet, simply at reduced speeds. If the reduced speeds are not enough for your needs, you can either upgrade your subscription, or top up your Turbo through the accounts portal.